Role of the cerebellum in high stages of motor planning hierarchy.

Motor planning is not a monolithic process, and distinct stages of motor planning are responsible for encoding different levels of abstractness. However, how these distinct components are mapped into different neural substrates remains an open question. We studied one of these high-level motor planning components, defined as second-order motor planning, in a patient (R.G.) with an extremely rare case of cerebellar agenesis but without any other cortical malformations. Second-order motor planning dictates that when two acts must be performed sequentially, planning of the second act can influence execution of the first. We used an optoelectronic system for kinematic analysis to compare R.G.'s performance with age-matched controls in a second-order motor planning task. The first act was to reach for an object, and the second was to place it into a small or large container. Our results showed that despite the expected difficulties in fine-motor skills, second-order motor planning (i.e., the ability to modulate the first act as a function of the nature of the second act) was preserved even in the patient with congenital absence of the cerebellum. These results open new intriguing speculations about the role of the cerebellum in motor planning abilities. Although prudence is imperative when suggesting conclusions made on the basis of single-case findings, this evidence suggests fascinating hypotheses about the neural circuits that support distinct stages of the motor planning hierarchy, and regarding the functional role of second-order motor planning in motor cognition and its potential dysfunction in autism.NEW & NOTEWORTHY Traditionally, the cerebellum was considered essential for motor planning. By studying an extremely rare patient with cerebellar agenesis and a group of neurotypical controls, we found that high stages of the motor planning hierarchy can be preserved even in this patient with congenital absence of the cerebellum. Our results provide interesting insights that shed light on the neural circuits supporting distinct levels of motor planning. Furthermore, the results are intriguing because of their potential clinical implications in autism.